Why do some people think that Siddhartha and Vasudeva are brothers?

When Siddhartha becomes distressed by his son's rebellion, Vasudeva encourages him to listen to the river and reminds him that he, too, left his own father to begin his path through life. After the young boy runs away, Vasudeva brings Siddhartha to the river so that he can hear that the "great song of a thousand voices consisted of one word: Om— perfection." When Vasudeva sees the look of serenity and knowledge shining in Siddhartha's eyes, he knows that it is time for him to go. "I have waited for this hour, my friend. Now that it has arrived, let me go. I have been Vasudeva, the ferryman, for a long time. Now it is over. Farewell hut, farewell river, farewell Siddhartha." Vasudeva then departs for the woods and the unity of all things.